From U.S. Pat. No. 4,528,957 a configuration for detecting the voltage of the lambda probe is known which utilizes a counter-voltage source, the voltage of which is connected in opposition to that of the lambda probe. Two threshold voltages are referred to the counter voltage, namely an upper threshold voltage U.sub.O and a lower threshold voltage U.sub.U. The counter voltage is, for example, 450 mV and the threshold voltages are separated therefrom by 50 mV upwards or downwards, respectively. If the measured voltage reaches one of the two thresholds, the system switches from open-loop to closed-loop control.
In closed-loop control operation, control is effected in the direction of lean as long as the voltage measured is above a switch-over threshold voltage and control is effected in the direction of rich if it is below this voltage. As a rule, the switch-over threshold voltage is close to the counter voltage, for example 10 mV above it.
In the case of the example mentioned, the counter voltage is therefore 450 mV, the upper switch-on threshold voltage 500 mV, the lower switch-on threshold voltage 400 mV and the switch-over threshold voltage 460 mV. Each voltage value is permanently associated with a digital value. In the case of the example, these are assumed to be the values 130, 143, 117 and 132, respectively.
In order to be able to set the voltage values accurately, a high-precision reference voltage source is used, that is, not the conventional voltage stabilizer which supplies, among others, the usual logic voltage of +5V for electronic components. The voltage of the reference voltage source is divided by means of high-precision constant-temperature resistors in such a manner that the desired counter voltage is accurately obtained. The threshold voltages are referred to this accurately set counter voltage. In addition to the calibration with the aid of the precise resistors, a software calibration can also be effected so that the predetermined digital value for the counter voltage is accurately obtained.